Fermi-LAT observations of flaring activity from PKS 0346-27 and PKS 0735+17

ATel #15129; G. La Mura (LIP, Portugal), on behalf of the Fermi Large Area Telescope Collaboration
on 20 Dec 2021; 23:34 UT
Credential Certification: Giovanni La Mura (glamura@lip.pt)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed renewed gamma-ray activity from a source positionally consistent with the flat-spectrum radio quasar PKS 0346-27, also known as 4FGL J0348.5-2749 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 57.159 deg., Dec. = -27.820 deg. (J2000; Beasley et al. 2002 ApJS, 141, 13), and redshift z=0.991 (White et al. 1988 ApJ, 327, 561).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on December 19, 2021, with a daily averaged gamma-ray flux (E>100MeV) of (2.6+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). It also shows associated emission of nine high-energy (>10 GeV) photons, the most-energetic one having an estimated energy of 25 GeV. The flux increase corresponds to a factor of 60 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). The corresponding photon index is 1.68+/-0.05, and is significantly smaller than the 4FGL value of 2.18+/-0.02. This source entered into a persistent high state on November 6, 2021 (see ATels #15020, #15092) and the current flux enhancement represents the highest flux ever observed by LAT on a daily scale.

On the same day, enhanced gamma-ray activity was also observed from a source positionally consistent with the BL Lac object PKS 0735+17, also known as 4FGL J0738.1+1742 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 114.531 deg., Decl. = +17.705 deg. (J2000; Fey et al. 2015 AJ, 150, 58), and redshift z = 0.45 (Gattano et al, 2018 A&A, 618, A80). Preliminary analysis indicates that this source was in an elevated gamma-ray emission state, with a daily averaged gamma-ray flux (E>100MeV) of (0.6+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), corresponding to a flux increase of a factor of 10 relative to the average flux reported in 4FGL. The estimated photon index is 2.0+/-0.2, and is consistent with the 4FGL value of 2.03+/-0.01 within the uncertainties. This source was also observed during a flaring state on December 8, 2021, close to the best-fit localization of the IceCube neutrino alert IC211208A (ATel #15099). The current flux is consistent within the uncertainties to the peak flux measured on December 4, representing the highest levels ever observed for this source on a daily scale.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. For 4FGL J0348.5-2749, the light curve can be accessed via the Fermi-LAT Light-Curve Repository (LCR) at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.php?source_name=4FGL_J0348.5-2749. The Fermi-LAT contact person for this source is Bindu Rani ( binduphysics AT gmail.com ). For 4FGL J0738.1+1742, the light curve can be accessed via LCR at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/lcr/source.php?source_name=4FGL_J0738.1+1742. The Fermi-LAT contact persons are S. Garrappa (simone.garrappa AT desy.de) and S. Buson (sara.buson AT gmail.com). We encourage multifrequency observations of these sources.

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.